This invention relates to a glass substrate for use in a magnetic recording medium, such as, a hard disk, a magnetic recording medium having the glass substrate, and a method of manufacturing the magnetic recording medium and the glass substrate.
An interface between a magnetic head and a magnetic disk has become a key technology for improving recording capacity in a technical field for magnetically recording (writing) and reproducing (reading).
It is necessary to excessively reduce a flying height of the magnetic head which is float over a surface of the magnetic disk to improve recording density.
However, when the record/reproduce (write/read) operation is carried out in the known CSS (Contact Start Stop) method, the magnetic head often sticks to the magnetic disk with a low flying height of the magnetic head. Herein, it is to be noted that this phenomenon is generally called xe2x80x9chead stictionxe2x80x9d.
Suggestions conventionally have been made about a variety of texture techniques to prevent such stiction of the magnetic head. A representative suggestion has been made about a method of forming a surface of an Al/NiP plating substrate into a rough surface by mechanically polishing (mechanically texturing) the surface in Japanese Unexamined Patent Publication (JP-A) No. Sho. 62-273619.
Further, another suggestion has been made about a method of depositing a thin-film having the rough surface on a glass substrate by the use of the known sputtering process in Japanese Examined Patent Publication (JP-B) No. Hei. 4-62413 or a method of forming the rough surface by the use of the chemical etching process in Japanese Examined Patent Publication (JP-B) No. Hei. 7-101507 when the glass substrate is superior in flatness in comparison with an aluminum substrate.
In the meanwhile, a glide height recently has reached 1.2xcexc inch or less to improve the recording capacity.
However, the method of forming the texture, which has been conventionally suggested and described above, is the texture technique on the condition that the glide height is equal to about 8xcexc inch.
Therefore, even when the conventional texture forming method is applied for the recent magnetic disk which records (writes) and reproduces (reads) with the low flying height, it is difficult to obtain the magnetic disk which simultaneously satisfies sufficient electro-magnetic conversion characteristic and stiction preventing effect of the magnetic head.
In this case, the conventional glide height was equal to about 8xcexc inch. Therefore, the surface state (the surface morphology) of the magnetic disk (the substrate) could be sufficiently evaluated by the known thally step. Herein, the surface roughness is measured by scanning a contact needle having radius of several xcexcm (for example, 2.5 xcexcm) along the surface.
However, when the flying height becomes 1.2xcexc inch or less (1 inch=25.4 mm) which is recently required, it is difficult to judge whether or not the surface state of the glass substrate can realize prevention of the stiction of the magnetic head by the use of the conventional thally step.
It is therefore an object of this invention to provide a magnetic recording medium and a glass substrate for the magnetic recording medium which has a glide height of 1.2xcexc inch or less and which is capable of realizing high electro-magnetic characteristic (high memory performance) and high CSS durability characteristic.
Inventors have paid attention to specify the surface state of the glass substrate by the use of the interatomic force microscope (AFM) in order to evaluate the surface state of the glass substrate. This is because it is impossible to identify whether or not the surface state of the glass substrate is suitable, since the resolution is low in the conventional measuring method using the contact needle method.
Based upon the above-mentioned evaluation method, it has been confirmed that height and distribution (namely, variation of the height) of each projection of fine roughness, which are formed on a principle surface of the glass substrate, are important factors to achieve the above purpose.
Further, as a result of various experiments, it has been found out that the glass substrate surface as a target or a goal can not be obtained unless specific polishing condition and surface process condition are properly combined. This invention is performed base upon this analyzed result.
(First invention)
In the glass substrate for use in the magnetic recording medium according to the first invention, when the surface roughness of at least the principal surface of the glass substrate is measured by the use of the interatomic force microscope (AFM), Ra (center-line mean roughness) falls within the range between 0.2 and 2.5 nm, Rmax (maximum height) falls within the range between 3 and 25 nm, and Rmax/Ra falls within the range between 3 and 35.
(Second invention)
In the glass substrate according to the first invention, a surface parallel to a virtual principal surface is defined as a contour surface when it is assumed that the principal surface of the glass substrate is completely flattened,
when the roughness formed on the principal surface is cut by the use of the contour surface (a surface which is structured by the same height components), a percentage rate of a total area of the principal surface for sum of areas of the cut cross sectional surface is defined as a bearing ratio,
the contour surface having the bearing ratio of 50% is defined as a referential surface,
each distance from the referential surface to each contour surface having each bearing ratio is specified as a bearing height,
the bearing height of the contour surface having the bearing ratio of 2.5% is defined as B.H. (2.5) while the bearing height of the contour surface having the bearing ratio of 5.0% is defined as B.H. (5.0), and
B.H. (2.5)/Rmax falls within the range between 0.1 and 0.5 or B.H. (5.0)/Rmax falls within the range between 0.1 and 0.45.
(Third invention)
In the glass substrate according to the first invention, the surface roughness of at least the principal surface is measured by the use of the interatomic force microscope (AFM),
Rmax falls within the range between 5 and 25 nm, and
B.H. (2.5)/Rmax fall within the range between 0.1 and 0.5 or B.H. (5.0)/Rmax falls within the range between 0.1 and 0.45.
(Fourth invention)
In the glass substrate according to any one of the first through the third inventions, the glass substrate contains at least alkali metal oxide and alkali earth oxide, and
the content of alkali earth oxide is not exceeding 3 mol %.
(Fifth invention)
In the glass substrate according to any one of the first through the fourth inventions, the glass substrate is used for a magnetic disk which has a glide height having 1.2xcexc inch or less.
(Sixth invention)
In a magnetic recording medium according to the fifth invention, at least a magnetic layer is formed on the principal surface of the glass substrate of any one of the first through the fifth inventions.
(Seventh invention)
A method of manufacturing a glass substrate which is for use in a magnetic recording medium, comprising the steps of:
preparing a glass member in which Ra falls within the range between 0.1 and 1.0 nm when a surface roughness of a principal surface of the glass member is measured by the use of the interatomic force microscope (AFM), and
chemically processing the surface so that Ra falls within the range between 0.2 and 2.5 nm, Rmax falls within the range between 3 and 25 nm, and Rmax/Ra falls within the range between 3 and 35.
(Eighth invention)
A method of manufacturing a glass substrate which is for use in a magnetic recording medium, comprising the steps of:
preparing a glass member in which Ra falls within the range between 0.1 and 1.0 nm when a surface roughness of a principal surface of said glass member is measured by the use of the interatomic force microscope (AFM), and
processing at least the principle surface by the use of hydrofluosilic acid.
(Ninth invention)
A method according to the seventh or the eighth invention, further comprising the steps of:
polishing at least the principal surface by the use of abrasive materiel containing free abrasive grain having grain diameter between 0.3 and 3.0 xcexcm before the chemical surface process or the surface process due to the hydrofluosilic acid.
(Tenth invention)
In a method according to the ninth invention, the chemical surface process or the surface process due to the hydrofluosilic acid processes the surface so that a portion having relatively high remaining distortion forms an island or peak in remaining stress distribution which is generated at a portion of abrasive trace due to the abrasive grain in the polishing step of the glass member.
(Eleventh invention)
In a method according to any one of the eighth through the tenth inventions, the hydrofluosilic acid has concentration between 0.15 and 3.0 by weight %.
(Twelfth invention)
In a method according to any one of seventh through ninth inventions, the glass constituting the glass member contains at least alkali metal oxide and alkali earth oxide, and content of the alkali earth oxide is not exceeding 3 mol %.
(Thirteenth invention)
In a method according to the twelfth invention, the glass constituting the glass member contains SiO2 between 58 and 75 weight %, Al2O3 between 5 and 23 weight %, Li2O between 3 and 10 weight %, and Na2O between 4 and 13 weight % as main components.
(Fourteenth invention)
In a method according to the thirteenth invention, the glass contains SiO2 between 62 and 75 weight %, Al2O3 between 5 and 15 weight %, Li2O between 4 and 10 weight %, Na2O between 4 and 12 weight %, and ZrO2 between 5.5 and 15 weight % as main components, and
weight ratio of Na2O/ZrO2 falls within the range between 0.5 and 2.0 while weight ratio of Al2O3/ZrO2 falls within the range between 0.4 and 2.5.
(Fifteenth invention)
In a method according to any one of seventh through fourteenth inventions, the step of chemically processing the surface or a chemical reinforcement process (chemical strengthening process) is carried out after the surface process due to the hydrofluosilic acid.
(Sixteenth invention)
In a method of manufacturing a magnetic recording medium according to the sixteenth invention, at least a magnetic layer is formed on the principal surface of the glass substrate manufactured by the method according to any one of the seventh through the fifteenth inventions.
(Seventeenth invention)
A method of manufacturing a glass substrate for use in a magnetic recording medium, comprising the steps of:
specifying a surface state of said glass substrate to improve flying characteristic of a magnetic head at every kinds of the magnetic head within a specific range Ra, Rmax and Rmax/Ra measured by the use of the interatomic force microscope (AFM);
processing the surface of the glass substrate under various surface processing conditions;
determining the surface processing condition so that Ra, Rmax and Rmax/Ra measured by the use of the interatomic force microscope (AFM) after the process falls within the specific range; and
processing the surface of the glass substrate based upon the determined surface processing condition.
According to the first invention, when the surface roughness of at least the principal surface of the glass substrate are measured by the use of the AFM, Ra falls within the range between 0.2 and 2.5 nm, Rmax falls within the range between 3 and 25 nm, and Rmax/Ra falls within the range between 3 and 35.
Thereby, the glass substrate for the magnetic recording medium, such as, the magnetic disk has the glide height 1.2xcexc inch or less without the stiction of the magnetic head.
Further, the glass substrate satisfies further high CSS durability characteristic in the magnetic disk using the CSS method.
Herein, it is to be noted that Ra and Rmax are center-line mean roughness and maximum height specified by JIS B0601, respectively.
The value of Rmax measured by the AFM represents the maximum height of the roughness formed on the substrate surface. When Rmax is not exceeding 5 nm, the substrate surface substantially becomes the mirror surface. This is not preferable because the magnetic head is stuck to the surface of the magnetic disk. When the Rmax exceeds 25 nm, the glide height exceeds 1.2xcexc inch. This is not also desirable.
The ratio of Rmax/Ra measured by the AFM represents distribution (variation (uniformity)) regarding height of the island (the peaks) in the roughness on the substrate surface and changes the coefficient of friction. Therefore, this ratio is an important factor.
Inventors have found out that the variation regarding the height of the island (peaks) of the roughness has a proper range to keep the glide height of 1.2xcexc inch or less and to realize the high CSS durability characteristic.
When Rmax/Ra which represents the height distribution (variation) of the peaks is not exceeding 5, the roughness relatively becomes uniform, the static coefficient of friction exceeds 3, and contacting area with the magnetic head is increased to readily occur the stiction. This is not preferable because the CSS durability characteristic is degraded.
Further, when Rmax/Ra exceeds 35, the maximum height of the peaks for the total average surface roughness becomes large. Thereby, the static coefficient of friction becomes 1 or less.
Although the contacting area with the magnetic head is reduced, a load for the maximum peak becomes large. This is not desirable because the head crash occurs.
The record/reproduce (write/read) method of the magnetic disk comprises the CSS method and the load-unload (namely, lamp load) method. For example, the surface roughness is controlled within the range specified by Ra=0.7xcx9c1.6 nm, and Rmax=8xcx9c18 nm, Rmax/Ra=10xcx9c20 in the CSS method.
On the other hand, the surface roughness is specifically controlled within the range specified by Ra=0.2xcx9c2.5 nm, Rmax=3xcx9c10 nm, and Rmax/Ra=3xcx9c15 in the load-unload method.
As mentioned before, Rmax/Ra is a parameter which indicates the distribution (variation (uniformity)) of the island (peak) of the roughness on the substrate surface. It is certain to be able to obtain the magnetic disk which has the glide height of 1.2xcexc inch or less and which satisfies the high CSS durability characteristic by selecting the predetermined range.
However, the projection (peak) on the surface of the magnetic disk, which directly contacts with the magnetic head, is practically a high projection (peak) corresponding to Rmax which is the maximum projection(peak) height among the roughness on the substrate surface. To control ratio (distribution) of the high projections (peaks) is important to realize further low flying height and obtain the high CSS durability characteristic.
Therefore, the ratio of B.H./Rmax as the parameter, which represents the ratio of the high projection (peaks) corresponding to Rmax, becomes excessively important.
According to the second and third inventions, the contour surface having the bearing ratio of 50% is defined as the referential surface, and the height from the referential surface is specified as the bearing height. Herein, it is to be note that the contour surface means a surface which is structured by the same height components.
When the bearing height of the contour surface having the bearing ratio of 2.5% is defined as B.H. (2.5) while the bearing height of the contour surface having the bearing ratio of 5.0% is defined as B.H. (5.0).
In this event, B.H. (2.5)/Rmax falls within the range between 0.1 and 0.5 or B.H. (5.0)/Rmax falls within the range between 0.1 and 0.45. This is preferable because the low flying height is further realized and the high CSS durability characteristic is obtained.
Herein, description will be made about the bearing ratio and the bearing height.
In the roughness picture illustrated in FIG. 1, the surface roughness on the principle surface of the glass substrate obtained according to this invention were measured by the use of the interatomic force microscope (AFM).
Further, the surface roughness on a specific linear line on the surface illustrated in FIG. 1 is represented in the measuring curve illustrated in FIG. 2.
Now, when it is assumed that the principal surface of the glass substrate is completely flattened, the virtual principal surface is defined and the surface parallel to the virtual principal surface is defined as the contour surface.
The contour surface, in which the distance from the virtual principal surface falls within the predetermined range, cuts roughness formed on the principal surface of the glass substrate.
A percentage rate of a total area of the virtual principal surface for sum of areas of the cut cross sectional surface of the all roughness cut by the contour surface is defined as the bearing ratio.
Further, the contour surface having the bearing ratio of 50% is defined as the referential surface and each distance from the referential surface to the contour surface having each bearing ratio is specified as the bearing height.
In this event, the above-mentioned bearing ratio is plotted in the abscissa axis, and the vertical distance from the maximum point of the contour surface, namely depth (bearing depth) is plotted in the vertical axis. This is called as the bearing curve.
In FIG. 3, the bearing curve of the surface roughness illustrated in FIG. 1 is illustrated. Herein, the contour surface, which is laid in the depth having the bearing ratio of 50%, is defined as the referential surface.
Further, the bearing height having the bearing ratio of 2.5% is defined as B.H. (2.5) while the bearing height having the bearing ratio of 5.0% is defined as B.H. (5.0).
In this case, when B.H. (2.5)/Rmax falls within the range between 0.1 and 0.5 or B.H. (5.0)/Rmax falls within the range between 0.1 and 0.45, it has been found out to realize further low flying height and obtain the high CSS durability characteristic.
The ratio of B.H./Rmax measured by the AFM relatively represents the rate (distribution) between the height in one bearing value and the maximum projection height, and deeply relates with the projection density.
The inventors have found out as a result of a variety of experiments that the value of the B.H./Rmax in the bearing ratio of 2.5% and 5% closely relates with the CSS durability and the static coefficient of friction. Namely, when B.H. (2.5)/Rmax and B.H. (5.0)/Rmax are not exceeding 0.1, respectively, the coefficient of friction due to sliding is increased and the CSS durability is lowered.
Further, when the B.H. (2.5)/Rmax and B.H. (5.0)/Rmax exceeds 0.5 and 0.45, respectively, the coefficient of friction exceeds 3. This is not preferable.
In this case, the surface having the bearing ration of 50% in the bearing curve is defined as the referential surface. However, this defines the referential surface of the bearing height, and the value of the arbitrary bearing ratio in the bearing curve may be adopted as the referential surface.
However, when the bearing ratio is 50%, it is preferable because the referential surface is a central surface between the island (peaks) and the concave portion in the roughness on the principal surface of the glass substrate.
Further, the value of the bearing height (B.H.) is selected to the bearing ratio 2.5% and 5% from the following reason. Namely, the density ratio of the island (peak), which has the height corresponding to the maximum surface roughness that contacts with the magnetic head in the corresponding relationship with the low flying height required for the magnetic disk, the low coefficient of friction and the high CSS durability, can precisely identified.
In Table 1, a plurality of (four) magnetic disks, which had the predetermined density of the island (peaks) measured by the use of the AFM, were prepared. Herein, the number of the islands (peaks) was 380 in the disk A, 96 in the disk B, 64 in the disk C, and 0 in the disk C when the disk was sliced with 6 nm from the center line of the surface roughness. In this condition, B.H./Rmax was calculated when the bearing ratio was set to 0.025%, 0.25%, 2.5%, and 25%, respectively. Table 1 represents the calculated results.
B.H./Rmax in Table 1 relatively represents the ratio of the island (peaks). However, when the bearing ratio was set to 0.025% and 0.25%, the relationship of the disk A greater than  the disk B greater than  the disk C greater than  the disk C must be naturally satisfied proportional to the density of the islands (peaks). However, the value of B.H./Rmax of the disk C becomes large in comparison with the disk A or the disk B.
This is because the rate of the number of the abnormal projections (peaks), which does not relate with the glide characteristic, is contained much, since the bearing ratio is considered as the relatively small value, namely, as only excessively small region of the islands (peaks). Therefore, it is not preferable in this case because the rate of the islands (peaks) can not be identified.
When the bearing ratio is 25%, the rate of the density of the islands (peaks) can be identified. However, it is not desirable because the range of B.H./Rmax between the respective disks is small.
The above-mentioned results are briefly explained as follows, and the second invention and the third invention depend upon these results.
(1) The bearing curve of the surface roughness of the substrate is determined, and the referential surface as the reference of the bearing height is defined. In the second invention and the third invention, the depth having the bearing ratio of 50% is selected as the reference.
(2) The bearing ratio is selected so that the specific parameter, in which the bearing height B.H. and the rate of Rmax measured from the referential surface determined from the above-mentioned (1) indicates the characteristic required for the magnetic disk or the state of the surface roughness in the corresponding relationship with this characteristic, can be precisely identified by the value of B.H./Rmax.
In the second invention and the third invention, the bearing ratio value (2.5%, 5.0%) is selected so that the rate of the islands (peaks) on the substrate surface in the corresponding relationship with the glide height, the coefficient of friction, and the CSS durability as the characteristic required for the magnetic disk can be precisely identified.
(3) The range of B.H./Rmax which satisfies the characteristic required for the magnetic disk is determined for the bearing ratio selected by the above-mentioned (2).
In the second invention and the third invention, the glide height is 1.2xcexc inch or less, the coefficient of friction is 3 or less, and B.H. (2.5)/Rmax which satisfies the excellent CSS durability falls within the range between 0.1 and 0.5 or B.H. (5.0)/Rmax falls within the range between 0.1 and 0.45.
The glass substrate for the magnetic disk manufactured by the method of controlling the surface roughness is useful for the magnetic disk in which it is necessary to strictly control the surface roughness of the substrate to achieve further flying height of the magnetic head.
According to the fourth invention, the above-mentioned glass substrate contains at least alkali metal oxide and alkali earth oxide, and content of the alkali earth oxide is not exceeding 3 mol %. Thereby, the desired surface roughness mentioned in the first through third inventions can be readily obtained by the chemical surface process (hydrofluosilic acid process) which will be described later.
According to the fifth invention, the magnetic disk, which is recorded (written) and reproduced (read) with the glide height of 1.2xcexc inch or less, is used. Thereby, the effect is maximized in the high recording density and the high CSS durability.
According to the sixth invention, at least magnetic layer is formed on the principal surface of the above-mentioned glass substrate. Consequently, the magnetic disk satisfies the high electro-magnetic conversion characteristic and the high CSS durability characteristic.
According to the seventh invention, the glass member, in which, Ra falls within the range between 0.1 and 1.0 nm when the surface roughness of the principal surface is measured by AFM, is prepared. The surface roughness of at least the principal surface of the glass member is chemically surface-processed so that Ra falls within the range between 0.2 and 2.5 nm, Rmax falls within the range between 3 and 25 nm, and Rmax/Ra falls within the range between 3 and 35.
Thereby, the glass substrate for the magnetic disk, which satisfies the high CSS durability, can be stably manufactured.
According to the eighth invention, the glass member, in which Ra falls within the range between 0.1 and 1.0 nm when the surface roughness of the principal surface is measured by the AFM, is prepared. At least the principal surface of the glass member is processed the surface by the use of hydrofluosilic acid.
Thereby, the glass substrate for the magnetic disk, which satisfies the high CSS durability, can be stably manufactured. The surface roughness of the glass substrate before the surface process must be set to the desired roughness (Ra=0.1xcx9c1.0 nm) and chemical material for processing the surface must be selected to the hydrofluosilic acid in order to stably manufacture the glass substrate for the magnetic disk which satisfies the high CSS durability.
Inventors have found out that the surface roughness of the glass substrate before the surface process gives large effect for the height distribution (variation) of the islands (peaks) on the substrate surface which is finally obtained to stably manufacture the glass substrate for the magnetic disk of this invention which is required to be controlled the surface roughness with high accuracy.
Inventors have enthusiastically researched this case. As a result, it is preferable that the surface of the glass substrate before the surface process is in the mirror state. Specifically, it is found out that Ra falls within the range between 0.1 and 1.0 nm, more preferably, that Ra falls within the range between 0.1 and 1.0 nm, and the Rmax falls within the range between 1 and 20 nm.
Further, the hydrofluosilic acid used during processing the surface of the glass substrate of the this invention has weak etching force (slow etching rate) as compared to hydrofluoric acid solution which contains hydrofluoric acid or potassium fluoride and which is conventionally used as the etching liquid.
Consequently, it is possible to precisely control the surface roughness. Silicofluoric acid (H2SiF6) is typically used as the hydrofluosilic acid.
The hydrofluosilic acid process may contain the other acid (hydrofluoric acid, sulfuric acid, hydrochloric acid, nitric acid) and commercially available washing materiel (natural washing material, surfactant, alkali washing material) with fine quantity in order to enhance the etching (washing) effect.
Further, the process condition of the hydrofluosilic acid is mainly determined in dependency upon concentration of the hydrofluosilic acid, immersing time into the hydrofluosilic acid, temperature of the hydrofluosilic acid.
The hydrofluosilic acid is formed by dissolving the silicofluoric acid into water. The concentration of the hydrofluosilic acid indicates the concentration in which the silicofluoric acid is dissolved in the water.
The concentration and the temperature of the hydrofluosilic acid relate with the etching rate (the specific range will be explained later) while the immersing time into the hydrofluosilic acid relates with the obtained roughness and the process time of the step.
These process conditions of the hydrofluosilic acid mainly relates with the surface roughness Rmax. Specifically, the surface roughness Rmax becomes larger as the concentration of the hydrofluosilic acid is higher, the immersing time into the hydrofluosilic acid is longer, and the temperature of the hydrofluosilic acid is higher.
The process condition of the above-mentioned hydrofluosilic acid is suitably adjusted based upon the roughness of the formed surface roughness. However, it is preferable from controllability of the surface roughness that the immersing time into the hydrofluosilic acid falls within the range between 50 and 600 sec and the temperature of the hydrofluosilic acid falls within the range between 15xc2x0 C. and 60xc2x0 C.
According to the ninth invention, the glass member before the above surface process is polished at least the principal surface by the use of the polishing material containing the free abrasive grain having grain diameter between 0.3 and 3.0 xcexcm.
The grain diameter mainly relates with the surface roughness Ra. Namely, when the average grain diameter of the abrasive grain becomes large, the surface roughness Ra of the glass substrate after the hydrofluosilic acid process becomes large (however, the surface roughness Rmax is not almost changed in this time).
Preferable density of the island (peaks) and tip shape of the island (peak) contacting with the magnetic disk can be obtained by setting the grain diameter within the range between 0.3 and 3.0 xcexcm.
In consequence, the glass substrate for the magnetic disk has further high CSS durability characteristic.
When the grain diameter of the free abrasive grain is not exceeding 0.3 xcexcm, aggregation of the polishing material readily occurs, and further, much residue generates after the washing step. This is not preferable. When the grain diameter exceeds 3.0 xcexcm, the roughness after the etching becomes excessively large. This is also undesirable.
Moreover, cerium oxide (CeO2), alumina (Al2O3), colloidal silica (SiO2), iron oxide (Fe2O3), chromium oxide (Cr2O3), zirconium oxide (ZrO2), titanium oxide (TiO2) are exemplified as the free abrasive grain.
According to the tenth invention, the chemical surface process or the surface process due to the hydrofluosilic acid processes the surface so that the portion having relatively high remaining distortion becomes the island (peak) in remaining stress distribution which is generated at the portion of abrasive trace due to the abrasive grain in the polishing step of the glass member.
Inventors have discovered that the trace, along which the abrasive grain passes, tends to be formed as the island (peak) when the surface is processed by the hydrofluosilic acid after polishing by the polishing material containing the free abrasive grain.
Although this mechanism is not clear, the load in the polishing step by the free abrasive grain is applied to the surface of the glass substrate. Consequently, the network of Sixe2x80x94O is (systematically and) structurally changed, and nonuniformity occurs in the remaining stress distribution by the structural change.
As a result, the etching rate due to the hydrofluosilic acid becomes slow in the portion having relatively high remaining distortion. This is assumed to be the above-mentioned mechanism.
In the eighth through the tenth inventions, the above-mentioned phenonmenon due to the discovery is positively utilized. Thereby, the desired surface roughness state can be firstly obtained.
The concentration of the hydrofluosilic acid preferably falls within the range between 0.15 and 3.0 weight (the eleventh invention).
When the concentration of the hydrofluosilic acid is not exceeding 0.15 weight %, the etching effect or the washing effect for the glass substrate is lowered. Consequently, the desired surface roughness can not be obtained.
When the concentration exceeds 3.0 weight %, it is difficult to control the surface roughness with high accuracy because the etching rate became quick. Consequently, the glass substrate for the magnetic recording medium having stable quality can not be obtained. This is not preferable.
If the projection (peaks) is formed in the first through the third inventions, no limitation is imposed as to a kind, a size and a thickness of the glass substrate used in this invention.
As the material of the glass substrate, aluminosilicate glass, soda-lime glass, soda aluminosilicate glass, aluminoborosilicate glass, borosilicate glass, and silicate glass are exemplified. The hydrofluosilic acid has excellent controllability of the chemical etching for the aluminosilicate glass, and the surface roughness can be controlled with high accuracy.
Further, the glass substrate (the glass substrate for forming the island (peaks) by the mechanism in the tenth invention) used in the manufacturing method of this invention is preferably formed by the material which contains the alkali metal oxide and in which the total content of the alkali earth oxide (RO:MgO, CaO and the like) is not exceeding 3 mol % (the twelfth invention).
The composition ratio is preferably specified by 58xcx9c75 weight % of SiO2, 5xcx9c23 weight % of Al2O3, 3xcx9c10 weight % of Li2O and 4xcx9c13 weight % of Na2O as main components (the thirteenth invention).
Further, it is desirable that the glass does not contain alkali earth (metal) oxide, such as, CaO or MgO to remarkably form the island (peak) by the mechanism in the tenth invention.
In particular, it is preferable in the fourteenth invention that the glass substrate is an aluminosilicate glass which contains 62xcx9c75 weight % of SiO2, 5xcx9c15 weight % of Al2O3, 4xcx9c10 weight % of Li2O, 4xcx9c12 weight % of Na2O, and 5.5xcx9c15 weight % of ZrO2 as the main components, the weight ratio of Na2O/ZrO2 falls within the range between 0.5 and 2.0, and the weight ratio of Al2O3/ZrO2 falls within the range between 0.4 and 2.5.
The transverse bending strength is increased, the compressive stress layer becomes deep, the Knoop hardness is excellent, and the controllability of the etching in the surface process due to the hydrofluosilic acid is excessively superior by chemically reinforcing (chemically strengthening) the aluminosilicate glass.
Therefore, such an aluminosilicate glass is desirable. Herein, it is to be noted that N5 manufactured by HOYA CORPORATION is representative of the above-mentioned aluminosilicate glass.
Further, the surface process due to the above hydrofluosilic acid is performed twice. Moreover, the different hydrofluosilic acid concentrations are used in the respective steps. Thereby, the fine surface roughness on the substrate surface can be controlled.
It is preferable that the chemical reinforcement (chemical strengthening) process is carried out after chemical surface process or the surface process due to the hydrofluosilic acid (the thirteenth invention). Herein, the known chemical reinforcement (chemical strengthening) methods are used as the above chemical reinforcement (chemical strengthening) method without limitation.
For example, the low-temperature ion exchange method, in which the ion exchange is performed in the region which does not exceed the transition point temperature from the viewpoint of the glass transition point, is preferable. A fused salt used for the chemical reinforcement (chemical strengthening) includes potassium nitrate, sodium nitrate, nitrate mixed with them.
When the above surface process due to the hydrofluosilic acid is performed immediately after the glass substrate surface is chemically reinforced (chemically strengthened), the remaining distortion formed by the free abrasive grains on the glass substrate surface is buried in the stress of the chemical reinforcement by chemically reinforcing (chemically strengthening). This is undesirable because the surface roughness can not be controlled.
However, the same result as the above-mentioned case can be obtained by the interposing the polishing processing step due to the free abrasive grains between (immediately after the surface process due to the hydrofluosilic acid) the chemical reinforcement process step (chemical strengthening process step) and the surface process due to the hydrofluosilic acid as the chemical reinforcement step (chemical strengthening process step)xe2x86x92the polishing step due to the free abrasive grainsxe2x86x92the surface process due to the hydrofluosilic acid.
According to the sixteenth invention, at least the magnetic layer is formed on the principal surface of the glass substrate manufactured by the method of manufacturing the glass substrate for the magnetic recording medium, such as, the above-mentioned magnetic disk.
Thereby, the magnetic recording medium, such as, the magnetic disk satisfies the high electro-magnetic conversion characteristic and the high CSS durability characteristic.
According to the seventeenth invention, the surface roughness of the principal surface of the glass substrate is controlled so that Ra, Rmax which indicate the height of the roughness measured by the interatomic force microscope (AFM), Rmax/Ra which indicates the height distribution of the roughness.
In consequence, the glass substrate for the magnetic recording medium used for the magnetic disk, satisfies the high electro-magnetic conversion characteristic and the high CSS durability characteristic.
Alternatively, the surface roughness of the principal surface of the glass substrate is controlled so that the ratio (B.H./Rmax) between the bearing height (B.H.) of the contour surface and Rmax, or Ra, Rmax, Rmax/Ra, B.H./Rmax fall within the specific range, as described in the following structures (a) and (b). In the above contour surface, the bearing ratio, which indicates the rate (the distribution) of the projection having the height corresponding to the maximum projection height, has the specific value.
Thereby, the glass substrate for the magnetic recording medium used for the magnetic disk, also satisfies the high electro-magnetic conversion characteristic and the high CSS durability characteristic.
(a) A method of manufacturing a glass substrate which is for use in a magnetic recording medium and which has surface roughness Ra and Rmax, where Ra is representative of a center-line mean roughness, Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point,
a surface parallel to a virtual principal surface is defined as a contour surface when it is assumed that the principal surface of the glass substrate is completely flattened,
when roughness formed on the principal surface of the glass substrate are cut by the use of the contour surface, a percentage rate of a total area of the principal surface for sum of areas of the cut cross sectional surface is defined as a bearing ratio,
the contour surface having the bearing ration of 50% is defined as a referential surface, and
each distance from the referential surface to each contour surface having each bearing ratio is specified as a bearing height (B.H.), comprising the steps of:
specifying a surface state of the glass substrate to improve flying characteristic of a magnetic head at every kinds of the magnetic head within a specific range ratio (B.H./Rmax) of the bearing height (B.H.) and Rmax measured by the use of the interatomic force microscope (AFM);
processing the surface of the glass substrate under various surface processing conditions;
determining the surface processing condition so that B.H./Rmax measured by the use of the interatomic force microscope (AFM) after the process falls within the specific range; and
processing the surface of the glass substrate based upon the determined surface processing condition.
(b) A method of manufacturing a glass substrate which is for use in a magnetic recording medium and which has surface roughness Ra and Rmax, where Ra is representative of a center-line mean roughness, Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point,
a surface parallel to a virtual principal surface is defined as a contour surface when it is assumed that the principal surface of the glass substrate is completely flattened,
when roughness formed on the principal surface of the glass substrate are cut by the use of the contour surface, a percentage rate of a total area of the principal surface for sum of areas of the cut cross sectional surface is defined as a bearing ratio,
the contour surface having the bearing ration of 50% is defined as a referential surface, and
each distance from the referential surface to each contour surface having each bearing ratio is specified as a bearing height (B.H.), comprising the steps of:
specifying a surface state of the glass substrate to improve flying characteristic of a magnetic head at every kinds of the magnetic head within Ra, Rmax, Rmax/Ra, and a specific range ratio (B.H./Rmax) of the bearing height (B.H.) and Rmax measured by the use of the interatomic force microscope (AFM);
processing the surface of the glass substrate under various surface processing conditions;
determining the surface processing condition so that Ra, Rmax, Rmax/Ra, and B.H./Rmax measured by the use of the interatomic force microscope (AFM) after the process falls within the specific range; and
processing the surface of the glass substrate based upon the determined surface processing condition.